Para-xylene is a valuable chemical feedstock which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C8 aromatic fractions from these sources vary quite widely in composition but will usually contain 10 to 32 wt. % ethylbenzene (EB) with the balance, xylenes, being divided between approximately 50 wt. % meta and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization or selective adsorption (e.g., the Parex process).
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C8 aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed to useful products while simultaneously minimizing any conversion of xylenes to other compounds.
One known method for removing ethylbenzene from a C8 aromatic stream is by dealkylation in which the ethylbenzene is converted to benzene and ethylene, with the latter normally being hydrogenated to produce ethane. Another known method for removing ethylbenzene is by isomerization to produce additional xylenes, normally through the intermediate step of saturating the ethylbenzene to produce naphthenes. In the past, a single catalyst was used to effect both xylene isomerization and ethylbenzene conversion, but this necessarily involved compromising between the different catalytic requirements of the two reactions. More recently, processes have been developed which employ separate catalysts tailored specifically for the different catalytic functions.
For example, U.S. Pat. No. 4,899,011 describes a xylene isomerization process employing ethylbenzene dealkylation, in which a C8 aromatic feed, which has been depleted in its para-xylene content, is contacted with a two component catalyst system. The first catalyst component selectively converts the ethylbenzene by deethylation, while the second component selectively isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The first catalyst component comprises a Constraint index 1-12 molecular sieve, such as ZSM-5, which has an ortho-xylene sorption time of greater than 50 minutes based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120° C. and an ortho-xylene partial pressure of 4.5±0.8 mm of mercury, whereas the second component comprises a Constraint Index 1-12 molecular sieve which has an ortho-xylene sorption time of less than 10 minutes under the same conditions. In one preferred embodiment, the first catalyst component is ZSM-5 having a crystal size of at least 1 micron and the second catalyst component is ZSM-5 having a crystal size of 0.02-0.05 micron. Each catalyst component also contains a hydrogenation component, preferably a platinum group metal.
An improvement over the process of U.S. Pat. No. 4,899,011 is described in U.S. Pat. No. 5,689,027 in which the first catalyst component in the two component system is pre-selectivated by coking, or more preferably by deposition of a, surface coating of silica, to increase its ortho-xylene sorption time to greater than 1200 minutes under the same conditions as cited in the '011 patent. Using such a system it is found that high ethylbenzene dealkylation rates can be achieved with significantly lower xylene losses than obtained with the process of the '011 patent.
Although the first and second catalyst components of the systems described in U.S. Pat. Nos. 4,899,011 and 5,689,027 can be housed in separate reactors, these processes are usually practiced in a single reactor in which the different components form separate beds in, for example, a fixed, stacked bed reactor. In contrast, U.S. Pat. No. 5,705,726 describes a similar process in which the ethylbenzene dealkylation step is performed in a separate reactor from that used for the subsequent xylene isomerization step. In theory, such a two reactor system offers significant advantages over a stacked bed system in that it allows the operating conditions as well as the catalyst properties to be tailored for the different reactions involved. In this way, it should be possible to operate at high ethylbenzene conversion while the xylene isomerization step is conducted at the milder conditions necessary to minimize reduce xylene losses. In practice, however, two reactor systems have generally not been adopted at least in part because of the increased capital cost of installing a second reactor and associated equipment.
The present invention seeks to provide a process which allows xylene isomerization and ethylbenzene conversion to be conducted in separate reactors without significant increase in capital cost by utilizing space within an existing reactor to accommodate the xylene isomerization catalyst. In particular, the invention is based on the realization that the product from the xylene isomerization step is normally fed to a clay treater to effect removal of any trace olefins in the product and that recent advances in olefin removal catalysts have significantly reduced the amount of catalyst required in the clay treater. As a result the clay treater provides reactor space which is already available in a conventional aromatics plant and which is suitable for accommodating a xylene isomerization catalyst. In addition, since the clay treater is operated at mild conditions compared with those employed in conventional xylene isomerization processes, the xylene losses can be reduced to very low levels.